Make the action taken when a required header is missing configurable.

[DevL]

• given an atom -> look up the Plug.Conn.Status code.
• given an integer -> treat it as a status code.
• given a function -> invoke the function and pass it the conn struct and the missing header/connection key pair. Check the arity.
• given a content-type and a body, respond using that content-type and body.

[DevL]

Partially being addressed by #3.

[DevL]

Addressed by #4